1. Field of the Invention
The present invention generally relates to multi-beam light source units, multi-beam scanners and image forming apparatuses, and more particularly to a multi-beam light source unit which generates a plurality of beams in one optical scan, a multi-beam scanner which uses such a multi-beam light source unit to make the optical scan, and an image forming apparatus such as a laser printer, a digital copying machine and a facsimile machine which forms an image by use of such a multi-beam scanner.
2. Description of the Related Art
Recently, in image forming apparatuses such as laser printers and digital copying machines, a multi-beam scanner is employed to meet demands to realize a high recording speed and a high recording density. The multi-beam scanner simultaneously scans a scanning surface of a photoconductive body by a plurality of laser beams.
The multi-beam scanner collimates light beams from a plurality of semiconductor laser diodes, and composes the light beams with a minute mutual angular deviation along a sub scanning direction. The composed light beams are passed through a deflecting means and is imaged on the scanning surface by an imaging optical system in the form of a plurality of light spots, so as to scan a plurality of lines at one time. A semiconductor laser array having a plurality of light emitting points arranged in an array on a single substrate, is used as a light source of the plurality of laser beams.
For example, a Japanese Laid-Open Patent Application No. 8-136841 proposes a technique which adjusts a first optical path of a 2-beam laser diode array to a rotary axis of a rotary member, and rotates a second optical path about the first optical path.
On the other hand, a Japanese Laid-Open Patent Application No. 9-26550 proposes a technique which arranges a desired spot position within an inconsistency range in a depth direction of a plurality of light beams from a laser diode array.
Furthermore, a Japanese Laid-Open Patent Application No. 56-42248 proposes a technique which makes a light source having a plurality of light emitting parts rotatable about an optical axis of an optical lens, so as to adjust a beam pitch.
However, according to the system proposed in the Japanese Laid-Open Patent Application No. 8-136841, the first laser optical path is matched to the center of rotation of the rotary member, and the second laser optical path is rotated by turning the rotary member so as to adjust first and second scanning pitches (pixel density pitches). But because of the construction of this proposed system, the second laser optical path becomes arranged further away from a collimator lens than the first laser optical path. As a result, first and second beam waist positions on the scanning surface become different, thereby making it difficult to obtain a desired beam diameter.
In addition, in the semiconductor laser array having two or more light emitting points, it becomes even more difficult to obtain a desired beam diameter, and it is impossible to cope with the further improvement of the high-speed recording and the high-density recording.
On the other hand, by rotating the collimator lens about the optical axis of the collimator lens, it is possible to obtain a uniform beam pitch and a uniform beam shape with respect to each of the laser beams on an imaging surface side. But in this case, it becomes necessary to increase the precision of the dimensions of the constituent parts so as to match the optical axis of the collimator lens, and the cost of the parts increases. Moreover, when the number of constituent parts of the rotary member becomes large, an accumulation of the errors also becomes large. Consequently, the beam position on the scanning surface deviates from a desired position when the accumulated error is large and the rotary member is turned, and the cost of the system further increases because of the need to adjust and correct the beam position.